A Thermal View of the Roof

Last week we talked about how low-slope roofs leak and, as a result, how the water becomes trapped in the roof system—generally in the insulation—accelerating roof failure. According to a survey by the National Roof Contractors’ Association, on average, roofs in the United States are replaced every ten years or less!

A penetration to the membrane, such as this split along an expansion joint, is the point of entry for water that is then trapped in the roof system where it causes all kinds of trouble.

One of the major causes of roof failure is trapped moisture that rusts metal decks and fasteners, degrades insulation and membranes and leads to destructive freeze/thaw cycles. Since nearly all roofs leak at some point, you can’t stop the moisture from getting in, but you can find it and get it out, which will help you preserve your roof.

How do you find the moisture? There are three technologies that can be used. Capacitance and nuclear both sample only a small portion of the roof at a time and are therefore best used to verify moisture. Thermal imaging can quickly survey huge areas of the roof and, if the conditions are right, determine the location of trapped moisture right down to the inch! Sounds great, right? It is, but there are also constraints to using thermography.

The technology works best on roof systems with absorbent roof insulations like perlite, wood fiber, fiberglass or foam that has aged enough that it absorbs water significantly. We need to be able to image the absorbed water and, ideally, connect it with the point of entry through the membrane. Roofs with new foam insulation can still leak, but the trapped water is not “blotted up” in a readily detectable pattern.

When conditions are right, water trapped in the roof system can be seen thermally due to a difference in thermal capacitance or thermal conductivity. This area was a result of careless work by an HVAC tradesman.

Furthermore, many single-ply membranes—a common type of roof that often employ foam insulations—are quite reflective for the longwave imaging systems most of us use. It is that “double whammy” of reflection and low-absorbency that make many single-ply roofs tough to inspect.

However, the biggest hurdle may be that thermal surveys are dependent on the right weather conditions.Ideal conditions are a sunny day followed by a clear night with little wind. It is also possible to conduct an inspection at night or on a heavily overcast day if there is at least a 10°C (18°F) difference between the temperatures indoors and outdoors; for ballasted roofs there should be an 18°C (32°F) or greater difference. In all cases, the roof surface must be dry, including any gravel or ballast; if not, evaporating moisture will result in very confusing thermal patterns.

To inspect a built-up roof with absorbent insulation, inspect the roof at sunset or just after. Very clear patterns will typically reveal themselves about an hour later as the roof begins to cool, and will often remain visible for several hours. Thermography is an excellent tool to locate the “board edge” patterns you’ll see on these kinds of roofs.

Bald single-ply roofs tend to be quite reflective when seen through a longwave imaging system. Good results can still be had but only with greater care and more experience.

On single-ply systems, whether “bald” or ballasted, you need to aim for ideal inspection conditions and the results may not always

be as crystal clear as one would like. The images may come and go rather quickly because the insulation has less water trapped in it and, as a result, the thermal capacitance is not as great.